Alkene Formation during Low-Temperature Alkane Activation on Various Solid Acids or Superacids

Sommer, Jean; Hachoumy, Mohammed; Garin, F.; Barthomeuf, D.; Védrine, J.C.

doi:10.1021/ja00108a033

Cited by 70 publications

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“…Desorption takes place through a surface-concerted intermolecular deuteron transfer when an incoming reactant molecule is present in the vicinity leading to a new adsorbed tertiary trivalent cation on a neighboring site, which reinitiates the propagation of the H/D exchange between hydrocarbon and catalyst. [22] The H/D exchange of 2MP with the USY zeolite involves only protons on the carbon atoms adjacent to the tertiary carbon atom, independent of the accessibility of the sites. The regiospecificity of the H/D exchange at positions C1 and C3 can only be explained with alkene-type intermediates reprotonated according to Markovnikovs rule.…”

Section: Resultsmentioning

confidence: 99%

Low‐Temperature Alkane CH Bond Activation by Zeolites: An In Situ Solid‐State NMR H/D Exchange Study for a Carbenium Concerto

Haouas

Fink

Taulèlle

et al. 2010

Chemistry A European J

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Isotopic H/D exchange has been monitored by in situ MAS NMR spectroscopy of 2-[D(14)]methylpentane with H-USY to probe the controversy over the alkane conversion mechanism. The probe molecule has distinct exchangeable sites with different accessibility to the zeolite surface. In the early stages of the process, the regioselectivity of exchange demonstrates that the slow step of the mechanism is controlled by a carbenium ion intermediate. At a later stage of exchange, intramolecular hydride migrations, typical of carbenium chemistry, replace D by H also on other carbon atoms, resulting in a loss of regioselectivity. Therefore, the first and the subsequent steps of the H/D exchange proceed at this temperature through a carbenium intermediate species.

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Section: Resultsmentioning

confidence: 99%

Low‐Temperature Alkane CH Bond Activation by Zeolites: An In Situ Solid‐State NMR H/D Exchange Study for a Carbenium Concerto

Haouas

Fink

Taulèlle

et al. 2010

Chemistry A European J

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“…There are two reasonable possibilities to account for the promoting effect of Zn on H/D exchange: 1) the intermediate formation of an olefin [33,43] during dehydrogenation on Zn cations, representing the Lewis acid sites, or 2) the involvement of Zn alkyl species, formed by the dissociative adsorption of the alkane. [44][45][46] The first possibility only accounts for the regioselective exchange of propane and the exchange in ethane.…”

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Significant Influence of Zn on Activation of the C‐H Bonds of Small Alkanes by Brønsted Acid Sites of Zeolite

et al. 2008

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Herein, we analyze earlier obtained and new data about peculiarities of the H/D hydrogen exchange of small C(1)-n-C(4) alkanes on Zn-modified high-silica zeolites ZSM-5 and BEA in comparison with the exchange for corresponding purely acidic forms of these zeolites. This allows us to identify an evident promoting effect of Zn on the activation of C-H bonds of alkanes by zeolite Brønsted sites. The effect of Zn is demonstrated by observing the regioselectivity of the H/D exchange for propane and n-butane as well as by the increase in the rate and a decrease in the apparent activation energy of the exchange for all C(1)-n-C(4) alkanes upon modification of zeolites with Zn. The influence of Zn on alkane activation has been rationalized by dissociative adsorption of alkanes on Zn oxide species inside zeolite pores, which precedes the interaction of alkane with Brønsted acid sites.

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“…To rationalize the regioselectivity of the exchange process there has been general agreement [11,31,32,34] that with solid acids the mechanism (Scheme 1) is similar to that proposed for the reaction between isobutane and D 2 SO 4 , [39] that is, the reaction proceeds via carbocationic intermediates.…”

Section: Resultsmentioning

confidence: 83%

Room‐Temperature Alkane Reactivity in Zeolites: An H/D Exchange Study

Sido

Walspurger

Barbiche

et al. 2010

Chemistry A European J

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As evidenced by H/D exchange with acidic zeolites, isoalkanes react readily at room temperature whereas linear alkanes do not. The observed regioselectivity of the exchange process demonstrates that the main factor controlling the reaction is not the accessibility to the acid sites, but the intrinsic reactivity of the alkane. The mechanism is best rationalized by classic organic chemistry involving carbocationic intermediates including the Markovnikov rule.

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Alkene Formation during Low-Temperature Alkane Activation on Various Solid Acids or Superacids

Cited by 70 publications

References 0 publications

Low‐Temperature Alkane CH Bond Activation by Zeolites: An In Situ Solid‐State NMR H/D Exchange Study for a Carbenium Concerto

Low‐Temperature Alkane CH Bond Activation by Zeolites: An In Situ Solid‐State NMR H/D Exchange Study for a Carbenium Concerto

Significant Influence of Zn on Activation of the C‐H Bonds of Small Alkanes by Brønsted Acid Sites of Zeolite

Room‐Temperature Alkane Reactivity in Zeolites: An H/D Exchange Study

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